1. Field of the Invention
This invention relates to an apparatus and method for digitally decoding a tone signal and particularly a DTMF telephone dialing signal.
2. Discussion of the Prior Art
In many different applications information is communicated via a signal carrying one or more tones having frequencies within a predetermined set of frequencies. One of the most frequently occurring applications for such signaling is Touch Tone telephone dialing.
Touch Tone dialing uses a dual tone multi-frequency (DTMF) encoding technique in which the predetermined set of frequencies is established with four frequencies in a low band group of frequencies (697, 770, 852 and 941 Hz) and four frequencies in a high band group of frequencies (1209, 1336, 1477 and 1633 Hz). Each different telephone key (or alternatively each different data state or character) is represented by a signal including a unique combination of one tone from the high band and one from the low band. Sixteen different signal states may be represented by this encoding technique.
The traditional decoding technique for a DTMF signal requires the use of analog filters. First, all noise outside the predetermined frequency bands is eliminated and then the signal is separated into a high band group and a low band group of signals. Band pass filters corresponding to the four frequencies in each group are connected to the respective group signals. A different detector is coupled to each band pass filter to determine whether or not the frequency component corresponding to the attached filter is present.
A DTMF tone signal is detected as having a valid signal state when exactly one tone is detected in each signal group for a predetermined minimum time duration. If too many or too few tones are detected the detection criteria is not met and a valid signal state is not indicated. When a valid signal state does occur, the particular combination of tones is decoded to produce an indication of the corresponding key or signal state.
While providing satisfactory operation, analog filters tend to be expensive and bulky, particularly at the relative low frequencies that are typically involved. With the current widespread use of digital signal communication and processing it becomes desirable to substitute digital decoding for the analog filters. Digital decoding becomes particularly desirable when a signal is already represented in digital form. A digital signal is typically divided into fixed length frames, each containing a predetermined number of sample data points. To improve the dynamic range of the sampled signal, either an A-law of a .mu.-law coding technique may be used to represent the sample signal points.
Boddie, J. R., Sachs, N. and Tow, J., "Digital Signal Processor: Receiver for Touch-Tone Service," The Bell System Technical Journal, Vol. 60, No. 7, (September 1981), pp. 1573-1583, describes a digital implementation of a typical analog filter decoder. The digital decoder uses digital filtering in place of the analog filters of the typical analog decoder. While providing satisfactory operation, this digital filtering technique consumes a great amount of digital signal processor (DSP) operating time.
Mitel, "An Introduction to Mitel DTMF Receivers," Application Note MSAN-106 (November 1981), pp. 3-3 to 3-9, describes another digital decoder. This system requires filtering to separate received signals into high and low bands and then uses a zero crossing count technique to detect tones in the two bands.
U.S. Pat. No. 4,354,248 to Conger et al, for "Programmable Multifrequency Tone Receiver," teaches the use of digital filtering to achieve multi-frequency decoding. A simplified filtering technique is implemented to reduce the number of required multiplications.
U.S. Pat. No. 4,379,347 to Girardi et al, for "Receiver for PCM-Encoded Multifrequency Dialing Signals," uses a decoding technique based upon a determination of the time duration between successive zero crossings of the received information signal.
U.S. Pat. No. 4,223,185 to Picou, for "Multifrequency Digital Signal Receiver," teaches a digital decoding technique using digital filters implemented with the fast Fourier transform (FFT).
U.S. Pat. No. 4,328,398 to Kawaguchi et al, for "Digital Multi-Frequency Receiver," discloses a digital MF detector using a digital filter coupled to filter each of the possible frequencies of interest. A large number of multiplications are required for the filtering process. Consequently, the arrangement is unsuitable for many low cost applications where the cost of the extensive data processing capability is not appropriate.